The present invention relates to a control apparatus for an electric power inverter (referred to generally in the following simply as an inverter) which performs conversion between DC and AC electric power. In particular the invention relates to a control apparatus for a 3-phase inverter having three pairs of series-connected switching devices respectively corresponding to the three phases, the inverter serving to operate a 3-phase synchronous rotary machine.
Harmonic frequency components, resulting from PWM (pulse width modulation) control of switching operations of switching devices of a three-phase inverter, are superimposed upon AC currents that flow between the inverter and the rotary machine. The harmonic components produce audible noise, by causing vibration of parts of the rotary machine and also cause increased switching losses. Hence it is desirable to reduce the effects of such harmonic frequency components.
One method which has been proposed for dealing with this problem, while using conventional 3-phase modulation is to increase the switching frequency (PWM carrier frequency) of the inverter above the audible frequency range, for example above 20 kHz. However raising the switching frequency results in an increased amount of switching losses.
To reduce switching losses, it is possible to employ 2-phase modulation. With 2-phase modulation, in each of successive modulation periods, the operating state of one of the switching device(s) respectively corresponding to the three U, V, W phases is held fixed, with PWM modulation being applied only to the other two phases.
However 2-phase modulation is ineffective in reducing the audible noise problem caused by the harmonic frequency components. There is thus a need for improved technology whereby the audible noise can be suppressed without significantly increasing the switching losses.